Resource reservation in transmission networks

ABSTRACT

The present invention provides for a reliable communication over radio networks co-existing in the same frequency band. According to the present invention, a transceiver operating according to another transmission scheme may transmit a frame in the IEEE 802.11 format. This frame may comprise an error. Upon receipt of this frame by an IEEE 802.11 station, this station may defer from channel access for a period of time.

BACKGROUND

The present invention generally relates to variable bit rate signals inthe field of communication systems and, more particularly, to decodingvariable bit rate signals which have been transmitted using one of aplurality of different coding rates.

The growth of commercial communication systems and, in particular, theexplosive growth of cellular radiotelephone systems, have compelledsystem designers to search for ways to increase system capacity withoutreducing communication quality beyond consumer tolerance thresholds. Onetechnique to achieve these objectives involved changing from systems,wherein analog modulation was used to impress data onto a carrier wave,to systems wherein digital modulation was used to impress the data oncarrier waves.

In wireless digital communication systems, standardized air interfacesspecify most of the system parameters, including modulation type, burstformat, communication protocol, etc. For example, the EuropeanTelecommunication Standard Institute (ETSI) has specified a GlobalSystem for Mobile Communications (GSM) standard that uses time divisionmultiple access (TDMA) to communicate control, voice and datainformation over radio frequency (RF) physical channels or links using aGaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rateof 271 ksps. In the U.S., the Telecommunication Industry Association(TIA) has published a number of Interim Standards, such as IS-54 andIS-136, that define various versions of digital advanced mobile phoneservice (D-AMPS), a TDMA system that uses a differential quadraturephase shift keying (DQPSK) modulation scheme for communicating data overRF links.

TDMA systems subdivide the available frequency band into one or more RFchannels. The RF channels are further divided into a number of physicalchannels corresponding to timeslots in TDMA frames. Logical channels areformed of one or several physical channels where modulation and codingis specified. In these systems, the mobile stations communicate with aplurality of scattered base stations by transmitting and receivingbursts of digital information over uplink and downlink RF channels.

The growing number of mobile stations in use today has generated theneed for more voice and data channels within cellular telecommunicationsystems. As a result, base stations have become more closely spaced,with an increase in interference between mobile stations operating onthe same frequency in neighboring or closely spaced cells. In fact, somesystems now employ code division multiple access (CDMA), using a form ofspread spectrum modulation wherein signals intentionally share the sametime and frequency. Although digital techniques provide a greater numberof useful channels from a given frequency spectrum, there still remainsa need to maintain interference at acceptable levels, or morespecifically to monitor and control the ratio of the carrier signalstrength to interference, (i.e., carrier-to-interference (C/I) ratio).

Another factor which is increasingly important in providing variouscommunication services is the desired/required user bit rate for data tobe transmitted over a particular connection. For example, for voiceand/or data services, user bit rate corresponds to voice quality and/ordata throughput, with a higher user bit rate producing better voicequality and/or higher data throughput. The total user bit rate isdetermined by a selected combination of techniques for speech coding,channel coding, modulation, and resource allocation, e.g., for a TDMAsystem, this latter technique may refer to the number of assignable timeslots per connection, for a CDMA system, this latter parameter may referto the number of assignable codes per connection.

Speech coding (or more generally “source coding”) techniques are used tocompress the input information into a format which uses an acceptableamount of bandwidth but from which an intelligible output signal can bereproduced. Many different types of speech coding algorithms exist,e.g., residual excited linear predictive (RELP), regular-pulseexcitation (RPE). etc., the details of which are not particularlyrelevant to this invention. More significant in this context is the factthat various speech coders have various output bit rates (referred to ascoding rates herein) and that, as one would expect, speech coders havinga higher output bit rate tend to provide greater consumer acceptance oftheir reproduced voice quality than those having a lower output bitrate. As an example, consider that more traditional, wire-basedtelephone systems use PCM speech coding at 64 kbps, while GSM systemsemploy an RPE speech coding scheme operating at 13 kbps.

In addition to speech coding, digital communication systems also employvarious techniques to handle erroneously received information. Generallyspeaking, these techniques include those which aid a receiver to correctthe erroneously received information, e.g., forward error correction(FEC) techniques, and those which enable the erroneously receivedinformation to be retransmitted to the receiver, e.g., automaticretransmission request (ARQ) techniques. FEC techniques include, forexample, convolutional or block coding (collectively referred to hereinas “channel coding”) of the data prior to modulation. Channel codinginvolves representing a certain number of data bits using a certainnumber of code bits. Thus, for example, it is common to refer toconvolutional codes by their code rates, e.g., ½ and ⅓, wherein thelower code rates provide greater error protection but lower user bitrates for a given channel bit rate.

Conventionally, each of the techniques which impacted the user bit ratewere fixed for any given service provided by the radiocommunicationsystem, or at least for the duration of a connection established by aradiocommunication system. That is, each system established connectionsthat operated with one type of source coding, one type of channelcoding, one type of modulation and one resource allocation. Morerecently, however, dynamic adaptation of these techniques has become apopular method for optimizing system performance in the face of thenumerous parameters which may vary rapidly over time, e.g., the radiopropagation characteristics of radiocommunication channels, the loadingof the system, the user's bit rate requirements, etc.

It is envisioned that many different combinations of these processingtechniques may be selectively employed both as between differentconnections supported by a radiocommunication system and during thelifetime of a single connection. However, the receiver must be aware ofthe types of processing being used by the transmitter in order toproperly decode the information upon receipt. Generally, there are twocategories of techniques for informing a receiver about processingtechniques associated with a signal: (1) explicit information, i.e., amessage field within the transmitted information having a mode valuethat is indicative of the processing type(s) and (2) implicitinformation, which is sometimes referred to as “blind” decoding,whereupon the receiver determines the processing performed by thetransmitter by analyzing the received signal. This latter technique isemployed in CDMA systems operating in accordance with the TIA/EIA IS-95standard. Explicit information is sometimes considered to be preferablebecause it reduces processing delay at the receiver, but comes at thecost of the need for the transmitter to include additional overhead bitsalong with the user data.

Of particular interest for the present invention are frame typeindicators which reflect the transmitter's currently employed source(speech) coding. As mentioned above, a frame type indicator may betransmitted to the receiver (whether it be the base or mobile station'sreceiver in a radiocommunication system, or any receiver in anonradiocommunication system) so that it can employ the appropriatespeech decoding techniques. Typically, this frame type indicator mayinclude just a few bits which are conveyed along with the data fields.Thus, it will be appreciated that it is particularly important for thereceiver to be able to accurately decode the frame type indicator since,otherwise, an entire frame of data may be unrecoverable. This desire foraccurate reception of the mode indicator has led designers to stronglyprotect such indicators with heavy channel coding to improve the chancesof accurately determining the correct speech decoding technique to beemployed.

However, usage of heavy channel coding implies higher redundancy, whichmeans more bits to be transmitted for the mode indicator field. This is,as explained earlier, undesirable since overhead bits should beminimized, not increased. Thus, it would be desirable to providetechniques and systems for increasing the likelihood that frame typeindicators will be properly decoded, while at the same time minimizingthe number of overhead bits which are transmitted with the payload data.

SUMMARY

These and other drawbacks and limitations of conventional methods andsystems for communicating information are overcome according to thepresent invention, wherein frame type indicators are included instead ofunused bits which were previously added to frames for rate adaptation.Thus, the frame type indicators can be relatively long, which providesrobustness against transmission errors. Moreover, different frame typeindicators may have different bit lengths.

According to exemplary embodiments of the present invention, a receivercorrelates the stored frame indicator patterns of different lengths,e.g., pseudorandom number sequences or more generally any type ofsequences which are at least partly orthogonal, with a received frame toidentify a frame type, e.g., a coding rate. A sufficiently highcorrelation provides a match such that the receiver identifies thereceived frame as having a rate corresponding to the frame typeindicator.

According to one exemplary method, the following steps are performedproviding at least two different coding rates for processing informationin a transmitter, wherein the frame type information is associated withsaid at least two different coding rates; coding, at the transmitter,information at a rate based on a selected one of the at least twodifferent coding rates; including a frame type indicator with the codedinformation in a frame, the frame type indicator being selected from atleast two frame type indicators depending upon the selected one of theat least two different coding rates, wherein the at least two frame typeindicators have a different bit length; and transmitting the frameincluding the frame type indicator and the coded information.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become more apparent upon reading from the followingdetailed description, taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a general block diagram of a radiocommunication system withinwhich frame type indicators according to the present invention can beimplemented;

FIG. 2 is a block diagram of a variable payload bit rate transmissionscheme having a fixed overall transmission rate;

FIG. 3 is a table illustrating different coding rates and bit types forwhich the present invention can be implemented;

FIG. 4 is an exemplary mapping of bit types to frames for the codingrates of FIG. 3;

FIG. 5 depicts a conventional technique for explicitly identifying aframe type;

FIG. 6 depicts identifying frame type according to an exemplaryembodiment of the present invention; and

FIG. 7 illustrates frame type indicators provided according to anotherexemplary embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well-known methods, devices, andcircuits are omitted so as not to obscure the description of the presentinvention.

The following exemplary embodiments are provided in the context ofradiocommunication systems. However, those skilled in the art willappreciate that this invention is applicable to any type ofcommunication system including wireline and wireless. Moreover, forsystems and methods wherein the present invention is applied to signalstransmitted over an air interface, the present invention is equallyapplicable, for example, to systems employing any access methodologyincluding frequency division multiple access (FDMA), TDMA, code divisionmultiple access (CDMA) and hybrids thereof.

Moreover, exemplary radiocommunication system operation in accordancewith GSM communication systems is described in EuropeanTelecommunication Standard Institute (ETSI) documents ETS 300 573, ETS300 574 and ETS 300 578, which are hereby incorporated by reference.Therefore, the operation of the GSM system is only described herein tothe extent necessary for understanding the present invention. Although,the present invention is described in terms of exemplary embodiments ina GSM system, those skilled in the art will appreciate that the presentinvention could be used in a wide variety of other digital communicationsystems which employ variable bit rate coding schemes.

Referring to FIG. 1, a communication system 10 in which exemplaryembodiments of the present invention may be implemented is depicted inorder to provide some context for this invention. Therein, system 10 isdesigned as a hierarchical network with multiple levels for managingcalls. Using a set of uplink and downlink frequencies, mobile stations12 operating within the system 10 participate in calls using time slotsallocated to them on these frequencies. At an upper hierarchical level,a group of Mobile Switching Centers (MSCs) 14 are responsible for therouting of calls from an originator to a destination. In particular,these entities are responsible for setup, control and termination ofcalls. One of the MSCs 14, known as the gateway MSC, handlescommunication with a Public Switched Telephone Network (PSTN) 18, orother public and private networks.

At a lower hierarchical level, each of the MSCs 14 are connected to agroup of base station controllers (BSCs) 16. Under the GSM standard, theBSC 16 communicates with a MSC 14 under a standard interface known asthe A-interface, which is based on the Mobile Application Part of CCITTSignaling System No. 7.

At a still lower hierarchical level, each of the BSCs 16 controls agroup of base transceiver stations (BTSs) 20. Each BTS 20 includes anumber of TRXs (not shown) that use the uplink and downlink RF channelsto serve a particular common geographical area, such as one or morecommunication cells 21. The BTSs 20 primarily provide the RF links forthe transmission and reception of data bursts to and from the mobilestations 12 within their designated cell. In an exemplary embodiment, anumber of BTSs 20 are incorporated into a radio base station (RBS) 22.The RBS 22 may be, for example, configured according to a family ofRBS-2000 products, which products are offered by Telefonaktiebolaget L MEricsson, the assignee of the present invention. For more detailsregarding exemplary mobile station 12 and RBS 22 implementations, theinterested reader is referred to U.S. patent application Ser. No.08/921,319, entitled “A Link Adaptation Method For Links usingModulation Schemes That Have Different Symbol Rates”, to Magnus Frodighet al. and filed on Aug. 29, 1997, the disclosure of which is expresslyincorporated here by reference.

According to exemplary embodiments of the present invention, informationtransmitted between a BTS 20 and a mobile station 12 can be processedusing different source (e.g., speech) coding modes. This information isthen, typically, forwarded through various links within the fixed partof the radiocommunication system. If the other party to the connectionis another mobile station 12, then the information is transmitted againover an air interface.

To more fully understand exemplary modes for which frame type indicatorscan be transmitted and decoded, consider the exemplary system portiondepicted in FIG. 2 wherein variable bit rate frames are mapped to afixed rate channel. Therein, frames of variable lengths Nr, generated bya variable bit rate coder 30 (disposed, for example, in a mobile station12), are mapped to a channel with a fixed rate, with a resulting framelength M. The coding rate r is determined by a control unit 32 andforwarded to the coder 30 which outputs speech frames consisting of Nrcoded bits per fame. These coded bits are then further processed andtransmitted over an air interface to a base station (not shown in thisFIG.). Once received in the fixed part of the network, supplementarydata, e.g., synchronization and other overhead information may be addedat block 34, which may be disposed, for example, in the transcoding unit(TRAU, not shown) of a BSC or MSC. Then, rate adaptation is performed atblock 36 (also disposed in the TRAU, for example) to selectively addmore bits to the output of block 34 to result in fixed rate frames oflength M being transmitted over digital line 38, or any type of digitalchannel, which may link TRAUs in different fixed nodes of theradiocommunication system.

FIGS. 3 and 4 provide an example of a system which designates fourdifferent coding rates which may be employed for informationtransmission. These coding rates and frame formats have been proposed inconjunction with the so-called tandem-free operation (TFO) systemdevelopment (IS-733). TFO systems are intended to reduce or eliminatemultiple transcodings associated with the forwarding of data blocksacross different interfaces. For example, instead of decoding speechframes received over the air interface from a mobile station,translating those decoded speech frames into pulse code modulated (PCM)speech samples for transmission over the communication links within thecommunication system and then recoding the speech samples once again fortransmission over another air interface to an intended recipient, TFOsystems are intended to transmit data blocks with only onecoding/decoding step in the nodes involved in forwarding informationbetween an originator and a recipient.

The table of FIG. 3 illustrates an exemplary relationship betweenpayload data Nr, supplementary data Dr and unused bits Ur which providefor a fixed frame size of, in this example, 320 bits (including CRCbits). The coding rates r specified therein are relative to a maximumoutput bit rate. These bits can be mapped into each frame in any desiredmanner, an example of which is illustrated in FIG. 4 for each codingrate. Those skilled in the art will recognize that the location ofparticular fields, e.g., payload data, supplementary data and unusedbits, can be adjusted as desired and that, in fact, these fields may bebroken up within each frame. As will be appreciated by reviewing FIGS. 3and 4, as the coding rate decreases, the number of unused, filler bitsincreases to maintain a fixed transmission rate.

As mentioned above, a frame type indicator can be transmitted in eachframe to inform the receiver's decoder to switch to an appropriate mode(e.g., rate 1, ½, ¼ and ⅛ in the foregoing example) to properly decodeeach frame. According to one proposal, the supplementary bits Dr can bereduced by a fixed number of bits F to permit transmission of the frametype indicator in the fixed number of bits F. This concept isillustrated in FIG. 5. However, the drawback to this proposal is that itreduces the number of supplementary bits available for overheadpurposes, e.g., synchronization, which in turn may result in a severedegradation in synchronization (and therefore system) performance.

According to exemplary embodiments of the present invention, a frametype indicator is instead created by making use of the unused bits Ur.More specifically, as seen in FIG. 6, a different frame type pattern isinserted into each frame in this exemplary system for rates ½, ¼ and ⅛rate frames. Since rate 1 frames do not include any unused bits, thoseframes need not have an explicit frame type indicator. Each frame typeindicator pattern can, therefore, have a different number of bits. Forexample, in this purely illustrative numerical example, a rate ½ frametype indicator can have up to 142 bits, a rate ¼ frame type indicatorcan have up to 221 bits and a rate ⅛ frame type indicator can have up to255 bits. The frame type indicator patterns can, for example, be createdby a pseudorandom number (PN) sequence generator in a manner which willbe apparent to those skilled in the art.

A receiver can then perform a pattern matching process to determine therate of a received frame. In this example, with four different codingrates, the receiver can attempt to identify a received frame byretrieving each of three known, frame type indicators from memory andsearching the received frame to determine if a match exists. Forexample, the receiver can retrieve a first frame indicator pattern of,for example, 142 bits that is associated with ½ rate frames anddetermine a correlation level between the received frame and the firstframe indicator. If the level of correlation is high enough, then thereceiver will identify that frame as a rate ½ frame. Otherwise, theprocess will continue to retrieve a second frame indicator pattern of,for example, 221 bits and perform a second correlation. If no match isfound, then the receiver will proceed to retrieve the third frameindicator pattern of, for example, 255 bits and perform a thirdcorrelation. If no match is identified, then the receiver will identifythe frame as a rate 1 frame, i.e., a frame without an explicit frametype indicator.

Alternatively, the received frame can be correlated with all of theframe indicator patterns. Then, the maximum correlation value can becompared with a threshold value. If the maximum correlation value isbelow the threshold, then the receiver identifies the frame with adefault (e.g., maximum) coding rate. Otherwise, if the maximumcorrelation value exceeds the threshold, then the frame is identified ashaving a coding rate associated with the frame indicator pattern thatgenerated the maximum correlation value.

Of course, even if all of the unused bits are used for the frame typeindicator, not all of the bits need to be involved in the correlation.For example, a subset of the stored frame type indicators can be used toperform the correlation depending upon the degree of accuracy desiredrelative to the processing resources to be employed in theidentification task.

The frame indicator field may, of course, be received with errors.Moreover, there exists the possibility that a certain frame typeindicator associated with rates=½, ¼, ⅛ will be transmitted or detectedinadvertently in a frame which was actually transmitted at codingrate 1. The probability of erroneous detection of the frame typeindicator in an error free case depends on the length of the frameindicator which, as discussed above, may vary for different coding ratessince the unused bits are used to convey the frame type indication. Withthe assumption that a pattern will be accepted as a match if it isreceived with up to 2 bit errors, the following formula gives theprobability P_(E) of an erroneously detected pattern, in case the bits(0, 1) are equally distributed and statistically independent:${P_{E}\left( {r \neq 1} \right)} = {\left( {1 + {Fr} + \begin{pmatrix}{Fr} \\2\end{pmatrix}} \right)/2^{Fr}}$This formula indicates that, for frame type indicators on the order of142 bits or greater, the probability of incorrectly identifying areceived frame's coding rate is negligible. However, if desired, thepayload and supplementary data can be screened prior to transmission todetermine if one of the frame type indicator patterns has randomlyoccurred. Upon recognizing that a frame type indicator pattern isincluded in the payload and/or supplementary bits, the transmitter canintentionally change one, two or more of those bits to prevent erroneouscoding rate identification at the receiver.

According to another exemplary embodiment of the present invention,variable rate source decoders can begin the decoding process once acertain number L_(r) of bits are available at their input. This numberof bits depends on the coding rate r, e.g., L₁=90 bits (for r=1), L₁₂=50bits (for r=½), L₁₄=45 bits (for r=¼) and L₁₈=20 bits. Then, if thenumber of bits in the frame indicator pattern is set to F_(r)=90−L_(r),the variable bit rate source decoder can determine the coding rate, andtherefore begin decoding, after 90 bits regardless of the coding rateemployed for a particular frame.

This concept is illustrated by way of example in FIG. 7 wherein thefirst 90 bits of a frame are illustrated for frames coded with each ofthe different coding rates used in the examples of this specification.Thus, for coding rate r=1, the first 90 bits contain only payload dataand no frame type indicator. For coding rate r=½, the first 90 bitsinclude a frame type indicator of 40 bits followed by 50 bits of payloadinformation. For coding rate=¼, the first 90 bits of the frame include a45 bit frame type indicator followed by 45 bits of payload information.Finally, for a frame of rate=⅛, the first 70 bits comprise the frametype indicator followed by 20 bits of payload information. Of coursethose skilled in the art will appreciate that the numbers provided inthis example are merely illustrative and that different number of bitscould be provided for the frame type indicators depending upon thevarying coding rates, etc.

Although the invention has been described in detail with reference onlyto a few exemplary embodiments, those skilled in the art will appreciatethat various modifications can be made without departing from theinvention. Although the frame formats described above portray thepayload, supplementary data and frame indicator fields as unitaryfields, any or all of these fields can be divided within the frame. Forexample, portions of the frame type indicator field can be interleavedwith portions of the payload data within each frame. Accordingly, theinvention is defined only by the following claims which are intended toembrace all equivalents thereof.

1. A method for transmitting frame type information in a communicationsystem comprising the steps of: providing at least two different codingrates for processing said information in said transmitter, wherein saidframe type information is associated with said at least two differentcoding rates; coding, at said transmitter, information at a rate basedon a selected one of said at least two different coding rates; includinga frame type indicator with said coded information in a frame, saidframe type indicator being selected from at least two frame typeindicators depending upon said selected one of said at least twodifferent coding rates, wherein said at least two frame type indicatorshave a different bit length; and transmitting said frame including saidframe type indicator and said coded information.
 2. The method of claim1, wherein a bit length of a frame type indicator associated with alower coding rate is greater than a bit length of a frame type indicatorassociated with a higher coding rate.
 3. The method of claim 1, whereinsaid coding is speech coding.
 4. The method of claim 1, wherein one ofsaid at least two coding rates are rate one and rate ½
 5. The method ofclaim 4, wherein said frame type indicator associated with rate one hasa bit length of zero and said frame type indicator associated with rate½ has a bit length of
 40. 6. A method for determining a frame type of areceived frame of information comprising the steps of: receiving saidframe; correlating said received frame with a first frame indicatorpattern; identifying said received frame as having a first typeassociated with said first frame indicator pattern if a result of saidcorrelation exceeds a threshold; correlating, if said received frame isnot identified as having said first type, said received frame with asecond frame indicator pattern; identifying said received frame ashaving a second type associated with said second frame indicator patternif a result of said correlation exceeds a threshold, wherein said firstand second frame indicator patterns have different bit lengths; andotherwise, identifying said received frame as having a third type. 7.The method of claim 6, wherein said first, second and third types aredifferent speech coding rates.
 8. A receiver comprising: receiveprocessing circuitry for receiving a frame of information; a memory forstoring a plurality of frame indicator patterns, including a differentframe indicator pattern for each of a plurality of different codingrates, at least two of said different frame indicator patterns having adifferent bit length; and a processor for correlating said frame ofinformation with each of said plurality of stored frame indicatorpatterns until a match is found to identify a coding rate associatedwith said frame of information.
 9. The receiver of claim 8, wherein eachof said plurality of stored frame indicator patterns has a differentlength.
 10. The receiver of claim 8, wherein a bit length of one of saidat least two frame type indicators associated with a lower coding rateis greater than a bit length of another of said at least two frame typeindicator associated with a higher coding rate.
 11. The receiver ofclaim 8, wherein said coding is speech coding.
 12. The receiver of claim8, wherein one of said at least two coding rates are rate one and rate½.
 13. The receiver of claim 12, wherein said frame type indicatorassociated with rate one has a bit length of zero and said frame typeindicator associated with rate ½ has a bit length of
 40. 14. A methodfor determining a frame type of a received frame of informationcomprising the steps of: receiving said frame; correlating said receivedframe with a plurality of frame indicator patterns; comparing a maximumcorrelation value, generated by said correlating step, with a threshold;identifying said received frame as having a first type associated with aframe indicator pattern that generated said maximum correlation value ifsaid maximum correlation value exceeds said threshold; and otherwiseidentifying said received frame as having a default type.
 15. The methodof claim 14, wherein said first and default types are different speechcoding rates.